A lithium secondary cell is a cell having a small size and a high energy density. In recent years, lithium secondary cells have attracted attention mainly as a power supply for a portable apparatus and a power supply for an electric vehicle in which a high capacity and a high output are necessary. From such a background, research and development on lithium secondary cells has been actively conducted in various countries, and the realization of a safe and high-capacity cell is imperative.
The lithium secondary cell includes an insulating layer that is disposed between a positive electrode plate and a negative electrode plate and has a function of electrically insulating the respective electrode plates and maintaining an electrolytic solution therein. The insulating layer has a property of easily contracting. Accordingly, in a case of keeping the lithium secondary cell in an extremely high temperature environment for a long time, the positive electrode plate and the negative electrode plate may come into physical contact with each other and thus an internal short-circuit may occur. In addition, the insulating layer may be broken due to a conductive particle (foreign matter) that is attached to the surfaces of the positive electrode plate, the negative electrode plate, and the insulating layer, and thus the positive electrode plate and the negative electrode plate may be electrically connected to each other and an internal short-circuit may occur. In particular, the recent development of lithium secondary cells has been done to realize a high capacity and thus there is a tendency for the insulating layer to be made to be thinner. As a result, the problem of the above-described internal short-circuit is becoming more important.
In addition, when an internal short-circuit between the positive electrode plate and the negative electrode plate occurs once, a short-circuit portion is further enlarged due to joule heat accompanying a short-circuit current and at the same time, abnormal heating occurs, and thus the cell may be broken. On the other hand, even when an internal short-circuit occurs in the cell, securement of safety by suppressing of the above-described breakage is very important. In recent years, research and development into a technology of raising safety under an internal short-circuit of the cell has been also actively conducted. For example, a technology in which an insulating layer formed from ion permeable ceramic particles and a binder is laminated on electrode plates (refer to PTL 1), or the like has been suggested.
On the other hand, even when an internal short-circuit occurs in the cell, it is very important to accurately evaluate the safety of the cell for securing safety of the cell in which an internal short-circuit occurs. In addition, in the related art, a cell evaluation test (safety evaluation items of cell such as a lithium secondary cell) that evaluates the operation of heat generation due to an internal short-circuit is established as UL standard (UL1642) for lithium cells, guidelines (JIS B8714) from cell industry society, and the like. Examples of such an evaluation test include a nail penetration test, a crushing test, and the like.
In the nail penetration test, a cell is pricked with a nail from the outside to cause short-circuiting of the positive electrode plate and the negative electrode plate by the nail, and the variation in temperature, voltage, or the like of the cell, which is caused by the joule heat generation that occurs, is measured (refer to PTL 2). As shown in FIG. 5, PTL 2 discloses a method in which nail 44 is made to penetrate through an insulation layer 412 and an insulation resistance is measured until nail 44 reaches current collector 416. According to this method, a pseudo internal short-circuit due to foreign matter is simulated. The crushing test is a test in which the cell is physically deformed by a round bar, a square bar, a flat plate, or the like to cause an internal short-circuit to occur between the positive electrode plate and the negative electrode plate, and then the variation in temperature, voltage, or the like of the cell is measured.
Furthermore, a cell having a structure in which a fracture mechanism is provided inside the cell so as to prevent smoking, ignition and the like, when occurring an internal short-circuit in single cell article is also suggested (refer to PTL 3 and PTL 4). As shown in FIGS. 6A-6B, this fracture mechanism is configured in such a manner that when an external pressure is applied to burrs 58 provided in the vicinity of housing 51 inside the cell, burrs 58 make a hole in part of separator 72. Accordingly, an internal short-circuit occurs between the outermost layer of positive electrode 60 and the outermost layer of negative electrode 50 of electrode assembly 30 that is spirally wound. Due to this fracture mechanism, in the above-described crushing test, burrs 58 that are provided on the negative electrode plate make a hole in separator 72, and thus the short-circuit is caused to occur between the positive electrode plate and the negative electrode plate. Accordingly, a cell voltage decreases so as to prevent smoking and ignition from occurring during the occurrence of an internal short-circuit.
However, for the cell provided with the fracture mechanism, a manufacturing process of the cell is difficult. In addition, when the fracture mechanism is provided, possibility of deterioration in safety of the cell itself cannot be denied. In addition, it is impossible to specify at which place of the cell an internal short-circuit due to intrusion of foreign matter occurs. Accordingly, even when the fracture mechanism is provided, it cannot be considered that the safety of the cell according to an internal short-circuit is high.
In addition, a cell in which a heater is embedded is disclosed. As this cell, for example, a secondary cell in which the heater is disposed in a sheet-shaped laminated structure is disclosed (for example, refer to PTL 5 and PTL 6). For example, the heater is disposed across substantially the entire surface of an insulating layer. In such secondary cell, a temperature of the entire inside of the cell is maintained at a desired temperature. According to this temperature control, a high output and a large discharge capacity may be obtained under a low-temperature environment.
Furthermore, as the cell in which the heater is embedded, a fuel cell, which is provided with a linear heater that heats an electrolyte, is disclosed (For example, refer to PTL 7). The heater is one linear heat generating body that is across an active cell surface of a fuel cell unit. The heater heats the electrolyte to a temperature equal to or higher than a freezing point of the electrolyte. According to this heating operation, only the electrolyte, which is immediately close to the linear heater, is heated. However, reaction heat of a cell reaction in the heated electrolyte further propagates to the neighboring electrolyte, and thus a heated region is broadened. In this manner, the fuel cell can quickly operate under a low temperature environment.
In addition, a heater is disclosed which includes a heat generating unit and a lead unit that are configured by a conductive ceramic material layer (for example, refer to PTL 9 and PTL 10). The heat generating unit is formed from a thin conductive ceramic material layer having a shape folded in a zigzag manner. The lead unit is formed from the conductive ceramic material in a thickness that is sufficiently larger than that of each conductive ceramic material layer. By this heater, a temperature of the heat generating unit may be raised to 1,000 to 1,500° C. within 10 seconds.